Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel. As a result, kinds of applications using the secondary battery are being increased owing to advantages of the secondary battery, and hereafter the secondary battery is expected to be applied to more applications and products than now.
Secondary batteries have different structures depending upon outputs and capacities required by applications and products, to which the secondary batteries are applied. For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDAs), digital cameras, and laptop computers, use one or several small-sized, lightweight cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, medium- or large-sized devices, such as electric bicycles, electric vehicles, and hybrid electric vehicles, use a battery module (or battery pack) having a plurality of unit cells electrically connected with each other because high output and large capacity are necessary for the medium- or large-sized devices. The size and weight of the battery module is directly related to the receiving space and output of the corresponding medium- or large-sized device. For this reason, manufacturers are trying to manufacture small-sized, lightweight battery modules. Furthermore, devices which are subject to a large number of external impacts and vibrations, such as electric bicycles and electric vehicles, require stable electrical connection and physical coupling between components constituting the battery module. In addition, a plurality of unit cells are used to accomplish high output and large capacity, and therefore, the safety of the battery module is regarded as important.
Generally, a medium- or large-sized secondary battery module is manufactured by receiving a plurality of unit cells in a case (housing) having a predetermined size and electrically connecting the unit cells with each other. A prismatic secondary battery or a pouch-shaped secondary battery, which can be stacked with high integration, is used as the unit cell. Preferably, the pouch-shaped secondary battery is normally used as the unit cell, because the pouch-shaped secondary battery is lightweight and has a relatively high safety.
However, the pouch-shaped battery, which is normally used as the unit cell of the battery module, has the following several problems in spite of the above-mentioned advantages.
First, the pouch-shaped battery has an electrode assembly mounted in a battery case made of a laminate sheet including a metal layer and a resin layer. For this reason, it is required to connect electrode taps in the battery case to corresponding electrode leads, thereby forming electrode terminals protruding to the outside, in order that the electrode assembly is electrically connected to the outside. As a result, the size of the pouch-shaped battery is increased. In addition, the electrode terminals are generally connected with the corresponding electrode terminals via wires, plates, or bus bars using welding. However, the welding operation is complicated and difficult.
Secondly, the electrode assembly of the pouch-shaped battery is mounted in the battery case in a sealed state by seaming an upper laminate sheet part and a lower laminated sheet part of the laminate sheet with each other. However, the electrode terminals must be bonded to the laminate sheet at some of the seamed region (sealing part) of the laminate sheet, at which the electrode terminals protrude to the outside. Consequently, even when using an additional polymer adhesive, the sealability at the seamed region is not sufficient. As a result, the electrode terminals and the laminate sheet may be easily separated from each other under harsh conditions, which results in the defectiveness of the battery.